An Introduction to the Aurora

The aurora is a beautiful, shimmering display of light from the upper atmosphere that is commonly seen during the night in polar regions of the Earth. The northern aurora is known as the aurora borealis and its southern counterpart is known as the aurora autsralis.

What causes the aurora? The sun emits a continuous stream of charged particles called the solar wind. This wind interacts with the magnetic field of the Earth and produces large electrical currents. These currents flow along the magnetic field lines down into the upper atmosphere surrounding the north and south magnetic poles (see diagram). These currents cause the atmospheric gases to glow like the gas in a fluorescent tube. A very large quantity of energy is deposited in the upper atmosphere during an auroral display. For example, about 1,000,000 kW, the equivalent to the power capacity of a large power plant, is needed to power a medium-strong auroral light of about 10 x 1,000 km. It should also be noted that only 1% of the energy of the precipitating particles actually goes into the production of visible light !

Where do aurora occur? Auroras occur around the Earth's north and south geomagnetic poles in regions known as auroral ovals. The auroral ovals are constantly in motion, expanding towards the equator or contracting towards the pole, and constantly changing in brightness. Occasionally the aurora may be seen as far south as the south of England. However, this would be a rare event - with maybe 1 or 2 displays every 10 years. This plot shows the typical location of the auroral oval during conditions of moderate activity. Click here to see what the weather is like in space today.

The gases in the aurora glow with characteristic colours. The emissions are the results of processes where atoms or molecules are excited by collisions with precipitating particles, returning to the ground state by emitting photons of a characteristic frequency or set of frequencies. This photograph shows a view of a green-rayed auroral arc. Green is the most common colour in the aurora and is caused by atomic oxygen at altitudes of between about 100 and 200 km.

This photograph shows a view of the aurora with a red and a green component. The red colour, seen predominantly in the left half of this frame, is due to atomic oxygen at altitudes of about 250 km.

This view of the aurora australis was taken by the Space Shuttle Discovery from low Earth orbit at an altitude of about 250 km. The variation of the colour of the glow of the aurora with height may be clearly seen with the red aurora occurring at greater heights than the green aurora. (NASA photo).

This image shows a view of the entire auroral oval taken by the Dynamics Explorer 1 satellite on 8 th November 1981 from high above the north polar region. This image was taken at an altitude of approximately 20,000 km. Here the glowing oval of auroral emission is about 4,500 km in diameter. Click here for the latest visible images from the Visible Imaging System also on Polar. (image courtesy of Louis A. Frank)

Does the aurora occur on other planets? The answer is yes! The Earth is not the only planet in the solar system on which auroras may be seen. Auroras may also be seen on the gas giants, Jupiter, Saturn, Uranus and Neptune. These planets all have strong magnetic fields and vast atmospheres.These images taken by the Hubble Space Telescope, show the aurora at Jupiter. These auroras happen for the same reason as on Earth. The gases in the upper atmosphere are excited by charged particles pouring in near the north and south magnetic poles. Most of the particles come from the solar wind but some flow from Jupiter's nearest moon, Io, and are trapped by the magnetic field. Click here for the official press release. (NASA photo)

Good starting points for further information on the aurora are available at www.geo.mtu.edu and www.exploratorium.edu and links therein.